1. Technical Field
The present invention relates generally to the field of computer manufacturing and more specifically to reducing delamination of, and cathodic/anodic filament growth on, boards used in computers.
2. Background Art
Computers and similar electronic equipment have become ubiquitous elements in the lives of people. Many businesses, banks, and governments rely on computers for their everyday activities. A large portion of the global community require that computers be reliable, stable facets of their economic, societal, and communication foundations. Computers today are required to run longer, with less down-time, than at any time in the past.
Because computers are so necessary, there has been an increased emphasis by computer designers on reliability. Many systems today cannot tolerate the extended down time necessary to replace failed components that make up the computer system. If each component is designed to last longer and be more reliable, then each computer, which is made exclusively of components, will last longer and be more reliable.
This emphasis on reliability of components has been applied to Printed Circuit Boards (PCBs). Most components in a computer system are designed by placing semiconductor packages or chips onto a PCB. PCBs are called “printed” because circuit runs or lines of copper are placed on the boards using techniques that were originally similar to the news print process. These circuit lines connect the semiconductor packages or chips together. PCBs can be as simple as an insulator that has lines printed on one or both sides and one or more components attached to one or both sides. PCBs are generally more complex, however, and are usually made of conductive, metal power and ground planes and several signal planes containing circuit lines sandwiched between layers of insulator, with metal lines and pads on the top and bottom surface of the sandwich. Top and bottom conductors are connected with each other and internal circuit layers using metal plated through holes (PTHs).
PCBs made in this manner have become the standard in electronics. Advances in manufacturing methods have made PCBs relatively inexpensive yet their simplicity makes them reliable. There are, however, problems associated with PCBs. One of the causes of some of these problems is water. The insulators in PCBs tend to be water permeable and to naturally absorb relatively high concentrations of water. Even if a PCB was dry when the component assembly process was completed, it may soon reabsorb water from humid air or through other processing steps. Thus, PCBs contain water, and this water freely permeates through insulating layers. Unfortunately, power and ground planes, which are usually made of copper metal, are not water permeable.
This lack of permeability affects PCBs and can cause failures. Water collects at the interface between a power/ground plane and the insulating layers, which sandwich the power/ground plane. The chips, chip carrier packages, or other components are soldered to the PCB (usually by wave soldering or infrared heat). These temperature increases can cause water that has collected at interfaces between the power/ground plane and the insulating layers to flash to steam. Water increases in volume dramatically as it becomes steam, and this expanding water/steam mixture can cause delamination of the insulator. In fact, “blisters” can appear in the surface of the insulator, leading to cracking of the insulator, line breakage, package ruptures, cracked PTH barrels, and other similar deleterious effects.
For the water to “escape” the confines of the insulator, the water must diffuse through the insulator to an area of lower water concentration. This area of lower water concentration generally only occurs at the periphery of the PCB including the top and bottom surfaces, where the laminate layers meet air. Assuming that the air actually has lower concentrations of water, diffusion of water through the dielectric into the atmosphere will occur over a long time. Until water has been removed from the PCB, however, the water can cause blister damage.
Another water-caused failure mechanism in PCBs is cathodic-anodic filament growth (CAF), which occurs when circuit board shorts grow along glass fibers. The shorts are formed when water leaches metal ions from adjacent conductors into the interface between a glass fiber and the dielectric. The copper ions are deposited when an electrical bias is applied; this deposition tends to form conductive dendrites. When the material is in solution, it is generally ionic so that it will migrate toward a metal feature that is oppositely charged. Cathodes are positively charged areas, while anodes are negatively charged areas. Thus, metal dendrites usually grow between two oppositely charged, local cathodic/anodic regions. These conductive metal dendrites then cause electrical shorts.
The failure mechanisms caused by water have been exacerbated somewhat by the use of PCBs for chip carriers. Chip carriers are devices to which chips are placed and connected before being connected to a board. In the past, these chip carriers were made almost exclusively of ceramics. Because of the use of ceramics for chip carriers, the Joint Electronic Device Engineering Council (JEDEC), a body organized to promulgate standards for electronic manufacturing, devised testing standards for chip carriers that essentially assume that the base substrate material absorbs no water at all. Now that PCBs have begun to be used in chip carriers, water migration and the problems associated therewith are more prevalent because there is simply more water in these organic materials. Chip carriers, which are made from organic laminate materials, are called laminate chip carriers (LCCs).
Therefore, without a way to limit failures caused by cathodic/anodic dendrite growth and delamination of insulators in organic LCCs, PCBs and LCCs will continue to have higher numbers of failures and reliability problems.